Bridge Approach Slabs 2

[BradPL]

I am curious to know others' thoughts on the design of bridge approach slabs...

I have been looking through some materials and there are a variety of techniques used to determine the reinforcing in the slab. Some examples have treated the approach slab as a simple span from the backwall to the approach pavement assuming no contribution from the subgrade below. Others pointed to relatively thin and lightly reinforced slabs available as standard drawings.

I ran through some simple span calcs and came up with extremely thick, heavily reinforced slabs (essentially the approaches are two additional slab spans). This seems a bit over conservative in my opinion, not to mention costly.

Does anyone have any thoughts on the contribution of the subgrade?

In the end I'm looking for a design that will handle the truck impacts without cracking and minimize the 'bump' up and down at the bridge ends.

 

[bridgebuster]

Most State DOT's have a standard design for approach slabs.

 

[Qshake]

Agree with BRIDGEBUSTER. Most DOTs developed some bridge approach pavement (25' or 30') back in the mid 1980s. Prior to that asphalt was common.

The ASCE Journal of Bridge Engineering last edition has a article on the behavior of the slab and it's design.

All-in-all, until the bridge approach slab is sitting solidly on supports which do not settle, you will not get rid of the "bump" experienced at bridge.

The bump comes from the abutment support being solid, supported on piles and the other end of the bridge approach pavement being supported on a "sleeper slab" or grade beam with no deep foundation. The sleeper slab settles, the bridge approach pavement rotates and the driver experiences lift-off.

At what point do you quit supporting pavement on pile - something will settle and off you go again!



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[10guage]

In Louisiana, we have standards for approach slabs. We use 40' approach slabs for fill conditions, 20' approach slabs for cut conditions and pile supported approach slabs where the soils are really bad.

There was a recent article in the Journal for Bridge Engineering about the poor performance of pile supported approach slabs in Louisiana primarily due to acceptance of a standard detail without some design judgement for site specific conditions such as pile downdrag.

In the past where I have had special situations, I've used a simple span assumption for design. In Louisiana, they are typically 12" thick.

Hope this helps.

 

[facca]

We have been placing approaches for years and I am not sure they are needed.On low volume roads I think the savings are justified.

 

[BradPL]

Thank you for your thoughts.

There is widely varying opinion on the function/necessity/behavior of these approach slabs. What we are aiming for is a longer, more heavily reinforced approach for interstate-quality roadways and a shorter, more economical version for low volume bridges.

As far as reinforcing goes, the bottom longitudinal mat is going to be based on partial undermining and the top longitudinal and both transverse mats will satisfy distribution requirements.

 

[eric1037]

The use of lightweight fill below the slab can help reduce the amount of settlement and, therefore, the bump. This could consist of geofoam, expanded shale, slag, flowable fill and other lightweight fill materials.